Obesity is the most prevalent health problem facing developed countries. Adiposity, a measure of fatness, is highly heritable. Studying genetic obesity disorders is important to the affected individual and helps in understanding normal metabolic pathways. The melanocortin pathway in the nervous system plays a central role in regulating food intake, energy expenditure, and metabolism. Mutations in the melanocortin 4 receptor (MC4R) are the most common form of monogenic obesity, found in 1-5% of obese people. We screened a cohort of severely obese patients undergoing Roux-en-Y gastric bypass (RYGB) surgery and found individuals with common and rare MC4R variants. There were no rare MC4R variants among control subjects. In the RYGB cohort, specific alterations in MC4R were associated with different weight loss outcomes. Given the strong association between rare MC4R variants and obesity, we believe that specific changes in cellular signaling by MC4R can affect weight loss. MC4R has traditionally been linked to cAMP signaling. Of the more than 100 different reported MC4R variants associated with obesity, many show normal ligand binding, expression and cAMP signaling. Therefore, the correlation between the in vivo obesity phenotype and cAMP signaling of the mutants is imperfect. In our preliminary data, we find that specific mutations and ligands can change relative signaling by MC4R to different pathways. We believe other signaling pathways downstream from MC4R and the resulting physiological output play critical roles in weight loss and weight gain. To test this hypothesis, the first aim is to stdy coupling of neuronal MC4R to different pathways when stimulated by various natural ligands. We will use quantitative, well-controlled approaches, to study native MC4R signaling to different pathways in neurons. In the second aim, we will test signaling by various naturally occurring mutants to each pathway, and determine the mechanisms that control their differential coupling. Using electrophysiological approaches, we will test how different signals regulate neuronal activity by modulating specific ionic currents. Finally, we will identify changes in gene expressio associated with each signaling pathway. These studies will identify MC4R signaling pathways that are critical in weight gain and will help identify new targets for specific therapies for obesty.